Part 24

Topaz

Topaz, an aluminum fluorosilicate, is a mineral especially prized by collectors because many specimens are gemstones. Topaz is transparent, has a glassy luster, and is quite hard (neither quartz nor a steel file will scratch it). The topaz that has been found in Texas is either colorless, pale blue, or sky blue. This mineral is fairly heavy—its specific gravity is 3.4 to 3.6. It cleaves perfectly in one direction (called basal cleavage), and some of the cleavage fragments have a flat, slabby appearance.

Topaz is commonly found as prism-shaped crystals, as cleavage fragments, and as irregular grains. Some fragments of topaz look like quartz. Topaz, however, is harder and heavier than quartz, and it has perfect basal cleavage, which quartz does not have.

In Texas, crystals, grains, and cleavage fragments of topaz occur in the Llano uplift area of central Texas. They are found near Streeter and Grit in west-central Mason County and near Katemcy in northern Mason County. Here, some of the topaz occurs in Precambrian pegmatite veins that cut through granite rocks. Most of the topaz, however, is found as pebbles in the gravels of nearby creeks, where it has washed after weathering out of the rocks.

Topaz probably originates when hot fluids move up out of molten magma into cracks and cavities in the surrounding rocks. There, the fluids react with elements in the rocks to form the topaz.

Topaz is a good gemstone because, in addition to its beauty, it is hard and is not easily marred by scratches. The Mason County topaz makes excellent gemstones. Most of it is beautiful and clear and is either colorless or of a pleasing blue color. These stones are cut, polished, and mounted in rings and other jewelry. A number of specimens of this Mason County topaz are displayed in museums.

Tourmaline

Tourmaline is a complex silicate of boron and aluminum. Other elements, such as magnesium, sodium, lithium, calcium, iron, or fluorine, also may be present. This mineral has a glassy to resinous luster. Only the dark-colored varieties of tourmaline have been found in Texas. One is a black variety called schorl, and another is a brown variety called dravite. Other kinds of tourmaline, although not found in Texas, are colorless or some shade of blue, yellow, red, pink, or green. Some crystals even show more than one color.

Tourmaline is too hard to scratch with a steel file, it has a specific gravity of 3 to 3.25, and it has a conchoidal to uneven fracture. Very little light passes through the dark varieties, and some fragments of schorl look like shiny, black coal.

Tourmaline occurs as masses without crystal shapes, but crystals are commonly found. The crystals are prism-shaped and have small vertical grooves, called striations, on the prism faces. When you look at some crystals from an end, you will see that the cross section is a triangle with the sides bowed outward.

Both the black and the brown varieties of tourmaline have been found at several places in the Llano uplift of central Texas. One well-known locality is at Town Mountain north of Llano in Llano County. Here, the tourmaline occurs in milky quartz that is associated with Precambrian granite rocks. In west Texas, in Culberson and Hudspeth counties, black tourmaline occurs in pegmatite rocks in the Van Horn Mountains, the Carrizo Mountains, and the Wylie Mountains. In the Eagle Mountains of Hudspeth County, it is found in metamorphic rocks as well as in pegmatites.

Some tourmaline formed from hot fluids containing boron that were given off by magmas far below the earth’s surface. These fluids traveled up through cracks and other openings in overlying rocks. As the fluids reacted with other elements and compounds, the tourmaline formed.

The clear, light-colored varieties of tourmaline are much admired, and they are more widely used as gemstones than are the dark-colored varieties. Some collectors, however, find that the dark-colored Texas tourmalines, when cut and polished, make shiny, attractive gemstones.

Some tourmaline is used as grinding material, but no Texas tourmaline is produced for this purpose.

Travertine. See Calcite.

Uranium Minerals (Carnotite, Uranophane, Pitchblende)

In 1945, the world suddenly became aware of the awesome power of atomic energy when the element uranium was used to produce some of the first atomic bombs. Uranium does not occur alone in nature but is found combined with other elements in a number of minerals.

All of the uranium minerals are radioactive. The uranium they contain is gradually breaking down and changing into a series of 13 other elements, called daughter elements. Each daughter element breaks down and changes into the next daughter element of the series. While breaking down, these elements give off particles and rays of energy.

This energy or radioactivity is made up of what are called alpha particles, beta particles, and gamma rays. You cannot see, hear, taste, smell, or feel them. The alpha and beta particles are weak and do not travel far. The gamma rays, however, can travel farther and can pass through seemingly solid material. Scientists have found that these rays can move through about 1 foot of rock, 2½ feet of water, and several hundred feet of air.

Prospectors searching for uranium minerals carry instruments that are able to detect this radioactivity. The uranium itself gives off only alpha particles, but some of its daughter elements give off gamma rays. These daughter elements are normally found with the uranium, and it is their strong gamma rays that the instruments are most apt to detect.

One of the instruments used is the Geiger counter. It indicates radioactivity by means of a meter, a flashing light, or a clicking sound, which can be heard through earphones. Another instrument for detecting radioactivity is the scintillation counter. It is more sensitive than the Geiger counter and it can detect radioactivity from a greater distance. The scintillation counter can be used from an automobile or an airplane, but the Geiger counter must be quite close to the source of radioactivity to be of use.

Various uranium minerals have been found, mostly in small amounts, in a number of places in Texas. Some of these minerals, such as uraninite or pitchblende, are heavy and dark colored. Others, including carnotite, tyuyamunite, autunite, and uranophane, are a shade of yellow or green. They are quite soft. Deposits of the light-colored uranium minerals have been mined from two areas of Texas. One of these areas is in Garza County on the Texas High Plains, and the other is in Karnes and Live Oak counties in the Gulf Coastal Plain.

One of the light-colored uranium minerals, carnotite, is a potassium-uranium vanadate, which has a bright canary-yellow or lemon-yellow color. This mineral is transparent to translucent and has an earthy or a pearly luster. Carnotite usually is found as crusts and as powdery masses. It is quite soft and can be scratched with a fingernail.

Carnotite, along with tyuyamunite, autunite, and several other soft, yellowish or greenish uranium minerals, is found in the Texas Gulf Coastal Plain. These minerals occur in the Jackson, Catahoula, and Oakville strata (which are Tertiary in age) in an area extending from Gonzales County to the Rio Grande (in parts of the area indicated by no. 2 and no. 3 on the geologic map, pp. 4-5). The largest deposits in this district have been found in the Karnes County area.

The Gulf Coastal Plain uranium minerals occur mostly with sandstones and clays in a sequence of strata that contains volcanic ash. It is believed that small scattered amounts of uranium compounds that were present in the volcanic ash sediments were dissolved by seeping underground water. These waters then moved into the sandstones and clays where they deposited the uranium as carnotite and as other uranium minerals.

Another uranium mineral, uranophane (calcium-uranium silicate), also occurs in Texas. Uranophane has a yellow to yellow-orange color and a pearly to greasy luster. When rubbed across a streak plate, it leaves a light yellow to a light yellow-orange streak. It is soft enough to be scratched by a copper penny. Uranophane has been found in extrusive igneous rocks in northwestern Presidio County in west Texas.

A dark-colored uranium mineral, pitchblende, is a variety of the mineral uraninite, uranium dioxide. Pitchblende does not occur with a crystal shape but rather as rounded and irregular-shaped masses. It is brownish black, greenish black, or black. If you rub it across a streak plate, pitchblende leaves a brownish-black streak. This mineral is heavy (it has a specific gravity of 6.5 to 8.5) and hard (a pocket knife will not scratch it, although a steel file will). Pitchblende has a submetallic luster and looks dull, greasy, or like pitch or tar.

Small amounts of pitchblende have been found at several places in Texas. One of these localities is a few miles west of Burnet in Burnet County in central Texas. Here, the pitchblende occurs in Precambrian igneous rocks that are associated with gneiss. In south Texas, some fine, scattered particles of pitchblende have been found about 325 feet below the surface in Tertiary (Pliocene) sediments that cover the Palangana salt dome in Duval County. No pitchblende is mined in Texas.